The effect of 1 MHz therapeutic ultrasound on the healing strength of tendons was studied in the tenotomized, repaired and immobilized right Achilles tendons of 26 rabbits. Twelve tendons were sonicated daily in continuous waves at a spatial averaged intensity of 1 W cm(2) [corrected] for 5 min. After nine consecutive treatments, the tendons were excised under anesthesia and compared biomechanically. Exposure to ultrasound induced a significant increase in both the tensile strength and the energy absorption capacity of the tendons. Although healing in rabbits may not translate directly to healing in humans, these findings suggest that surgically repaired human Achilles tendons may heal faster if ultrasound is applied during the early stages of healing.